Upstate New York Supercell Through Dual-Pol Radar

A monster supercell on May 22, 2014 produced giant, destructive hail and an EF-3 tornado just west of Albany, New York. In general, the event was poorly forecast.

day1otlk_20140522_1630_prt

The supercell formed outside of the “slight risk” area outlined by the Storm Prediction Center 1630 UTC forecast. Additionally the tornado probability was <2% and not contoured in the SPC forecast. The morning Area Forecast Discussion from Albany summed up the severe weather threat in the following way:

THEREFORE...WE FEEL THE THREAT OF SEVERE THUNDERSTORMS IS VERY LOW 
(BUT NOT ZERO PERCENT). THE BETTER THREAT APPEARS TO BE LARGE HAIL 
BUT AGAIN THAT IS A VERY LOW THREAT.

I don’t think anyone (myself included) expected a long-lived supercell producing a long path of significant severe weather. The storm gave us an opportunity to look at how this supercell looked with dual polarization radar – a relatively new tool in our severe weather arsenal.

The storm started out with a classic large hail signature in Fulton County, NY to the north of the New York State Thruway. The above video from the town of Broadalbin shows baseball size hail falling from the storm. While low level rotation was unimpressive the mid level mesocyclone was cranking.

Broadalbin2

From a dual pol perspective this was a classic large hail case. A large area of high reflectivity (near 70dbz in places) with ZDR near 0 and pockets of low CC. Because hail tends to tumble as it falls it appears spherical to the radar – hence a differential reflectivity near 0. Not shown is KDP which is fairly low over Broadalbin – only about 1º/km showing that liquid water was not a large contributor to the high reflectivity – the hail was! In addition, a three body scatter spike (note very low CC downraidial of hail core) was present from the lowest elevation slice (near 2,000 ft AGL) all the way up to 20,000 feet! Storm top divergence was also approximately 100 knots which is impressive for a northeastern US supercell.

amsterdam2

The supercell went on to produce large hail in the town of Amsterdam, NY as well. The largest hail fell in an area with somewhat low reflectivity (55dbz over Amsterdam as opposed to 65-70dbz farther east). This is another example of why dual pol variables are better at detecting large hail than using reflectivity alone!

Picca and Ryzhkov (2012) showed giant hail detection is possible by locating areas of wet hail growth above the freezing level, and in particular, in the hail growth zone near -15C. They cite CC <0.9 and ZDR <0 as clues. This event showed that signal over Amsterdam when giant hail was indeed falling. You can see very large Z near Amsterdam at 9500ft coincident with sub-zero ZDR and CC <0.9. This signature is also present up into the hail growth zone above the -10C isotherm.

wethailgrowth1

Rapid tornadogenesis took place following the large hail reports north of the Thruway. At 1928 UTC there is some sign of a reflectivity appendage at the lowest tilt. In addition the mid level mesocyclone is beginning to descend (not present at 0.5º or 0.9º tilts but is higher up at 1.3º). This was a tough event to warn for – as there weren’t many clues until the tornado was touching down.

By 1933 UTC tornadogenesis has occurred with a 56 knot gate-to-gate delta-V at 900 ft AGL. The tornado warning came out at 1939 UTC. By 1942 UTC a weak tornado debris signature is present and by 1951 UTC a clear tornado debris signature is present with very low CC, high Z, and near zero ZDR.

tds1

tds2

Given the strength of the tornado (EF-1 to EF-3), the proximity to the radar site, and the path of the tornado (through heavily wooded areas) I’m very surprised the TDS was not more dramatic. The “slam dunk” TDS only lasted 1 volume scan and only extended up to about 3500 ft AGL. Not particularly impressive.

One possible explanation for the relatively unimpressive debris signature was that there was a tremendous amount of rain and hail that was wrapped into the rear flank downdraft (KDP and Z quite high and ZDR near zero) that could have skewed the correlation coefficient higher than you’d typically expect with hydrometeors beings the dominant signal?

One other interesting feature is the presence of a ZDR arc shortly after tornadogenesis. The ZDR arc did not precede the tornado which frequently happens (no benefit for the radar operator) and was somewhat unusual because it appeared so rapidly.

zdrarc2

You can see the ZDR arc here at 1947 UTC along the reflectivity gradient of the storm’s forward flank. ZDR is quite high in places (nearly 7db) and there is clear separation between high ZDR and high KDP. This is an example of hydrometeor size sorting in a highly sheared environment. The high ZDR is indicative of large drops and the smaller drops are getting pushed farther into the storm’s forward flank where KDP shows high water content. The ZDR arc appeared as the tornado reached its maximum intensity (EF-3) in Duanesburg.

Courtesy: WNYT-TV

Courtesy: WNYT-TV

The sudden appearance of a ZDR arc might indicate that the storm was moving into a more strongly sheared environment where hydometeor size sorting occurred quickly. Not surprisingly, this is where the tornado was the strongest. At the time of tornadogenesis there is a region of high ZDR displaced well east of the low level mesocyclone but it is coincident with high KDP indicating little separation from size-sorting. Prior to and during tornadogenesis there was no real signal from a ZDR arc to help forecasters with this storm.

 

zdr4

 

The EF-3 tornado provided a unique opportunity to view the dual pol characteristics of this northeastern US supercell. Unfortunately, the storm’s rapid tornadogenesis was preceded with few, if any, clues on radar. When the tornado was on the ground the debris signal was surprisingly muted (many far weaker and farther from the radar tornadoes have produced much more dramatic TDS around here) but did eventually give forecasters confirmation of a tornado in the absence of any reports in real-time.

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Friday Severe Weather and Tornado Threat

I’ve been watching Friday’s severe weather potential with a bit of interest since Sunday. It hasn’t been clear whether ingredients would come together for severe weather with different computer models offering up very different outcomes. This afternoon, both the 18z NAM and GFS show the potential for a rather sizable severe weather event including tornadoes.

gfsNE_500_avort_021

The synoptic setup is pretty straightforward. A deep closed-low near the southern tip of Hudson Bay continues spinning with a number of shortwaves advancing east across the region. At the same time, mid level wind fields strengthen during the day tomorrow approaching 40 knots out of the west at 500mb while a weak wave of low pressure forms along a front to the west strengthening the southerly flow in the boundary layer.

gfsNE_850_spd_024

Now the GFS (the NAM has been showing this for a while) develops a strong low level jet over the region Friday. This creates a highly sheared environment in the 0-3km layer. Take a look at these hodographs from the 18z GFS and NAM for 18z tomorrow valid at 18z Friday.

HFDNAMandGFS

The long and curved hodographs are very impressive. In addition the models are developing some surface based instability tomorrow. The extent of the destabilization is very unclear, however.

18z GFS sounding for Hartford valid 18z Friday

18z GFS sounding for Hartford valid 18z Friday

So what will happen? This setup does seem like it could morph into a classic low CAPE/high shear severe weather day if the current depiction by the GFS and NAM comes to pass. Hail is quite unlikely but damaging winds and tornadoes are possible. The threat will depend on the amount of destabilization which is unclear and dependent on cloud cover and convection earlier in the day. That said, lifted condensation levels (LCLs) are quite low so any storm that begins rotating can produce a tornado.

While the NAM is likely overdone at 18z Friday – many of the parameters, especially the 70 j/kg of 0-3km CAPE and storm relative helicity >300 m2/s2, is sufficient for tornadoes and even significant tornadoes per Davies, 2006.

WVIT Severe Threat

We’ll have to watch tomorrow’s setup closely – I’ll keep you advised. If some of the model solutions turn out to be correct tomorrow may be a very busy last day of work before I leave for vacation!

Another EF-1 Tornado in Connecticut

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When Nick Stanczyc posted this picture on Instagram someone forwarded it to me within moments. I couldn’t believe it! The classic looking tornado was just about a half mile away from the intersection of Rt 30 and Rt 31 in Mansfield near Storrs.

On the 24th anniversary of one of the state’s most violent tornado outbreaks we got hit again – but by a much weaker storm. The first sign of trouble came around 1:30 when a rather unimpressive thunderstorm began to show signs of rotation over Watertown, Thomaston, and Plymouth.

watertown1

For a period of time while near Route 8 there was a Delta-V of 40 knots on adjacent gates in the storm’s radial velocity (i.e. 20 knots inbound and 20 knots outbound) about 4,000 ft above the ground. In addition, the reflectivity signature indicated a small hook echo or appendage associated with the storm. Unlike the July 1st storm (which was a low topped supercell) the rotation was seen through a large portion of the storm from about 4,000 ft at the lowest radar sample to 22,000 ft.

The 17z RAP analysis sounding shows an environment that was capable of producing rotating storms. The environment was quite unstable with surface based CAPE values exceeding 2000 j/kg. In addition there was fairly impressive directional and speed shear through the lowest 3km of the atmosphere. Because of this there was fairly sharp curvature of the hodograph between 1 and 2km above the ground. It’s no surprise that a somewhat discrete cell ahead of the main cluster was able to take on supercellular characteristics.

17zwatertburybufkit

While the radar presentation for the storm in Watertown was arguably the most impressive of the day the storm did not produce damage and did not produce a tornado. The likely reason why is that there was little 0-1km shear – the bulk of the shear was above that level. The height of the lifted condensation level or LCL (think of this as the cloud base) was around 800 meters. While low, this was not as low as we saw during the July 1st tornadoes or the subsequent storms in northeastern Connecticut (where LCLs were near 300 meters!)

The overall synoptic environment was characterized by a deep trough in the Great Lakes with a shortwave rotating underneath it through New York and New England. QG forcing for ascent was greatest ahead of this with fairly widespread shower and thunderstorm coverage.

gfsNE_500_avort_009

Shortly after 5 p.m. – 2 supercells had developed in Connecticut. One produced some damage near Tolland and another in Coventry and Mansfield. Here’s the 21z sounding from the RAP analysis at the Windham Airport. Most of the shear is located in the 0-1km layer with a helicity of 111 m2/s2. There is about 1100 j/kg of sb CAPE and LCL heights are VERY low – about 300 meters!

21zIJD

The storm in Tolland shortly after 4 was totally unimpressive on radar. There’s some signs of a weak low level mesocyclone but the storm itself was about as exciting on radar as a snow shower. Even so, it was apparently able to produce some damage in parts of Tolland just west of the Green as it crossed I-84. This damage was ruled to be from straight line winds and not a tornado with trees knocked down in a southwest to northeast fashion along the storm’s path.

The most likely “cause” for the wind damage in Tolland wasn’t a classic microburst but rather a damaging rear flank downdraft that produced a corridor of >50 knot southwesterly winds along the storm’s path. This would make sense given the fact many people saw a funnel cloud though there was no conclusive evidence of a touchdown.

tolland

As the storm moved northeast the mesocylone remained and even strengthened a bit out toward Willington and Union. It’s unclear to me what happened shortly after 5 p.m. The first mesocylone over Tolland weakened as it crossed into Stafford and Willington while a second, and stronger, mesocyclone develop on the southeast flank over Willington and then moving into Ashford and Union. This turned into the more powerful mesocyclone and peaked with a low level delta-V of about 40 knots and a tornado warning from the folks in Taunton. Whether this was simply the mesocyclone “cycling” or if this was a splitting supercell it’s hard to say based on the radar data I’m looking at now. Anyone have any ideas?

unioncycle

The most impressive storm of the late afternoon developed shortly after 5 p.m. to the southwest of the Willington/Union storm. Rotation was evident as the storm left Glastonbury and it strengthened in Hebron and Andover. The storm produced an EF-1 tornado from Andover to Mansfield with the worst damage concentrated in an area of Coventry near Coventry Lake.

storrsfinal

Given the super low LCLs and enough turning in the lowest kilometer of the atmosphere it’s not a surprise that this storm produced.

Given the advances in doppler radar (super resolution!) and the proliferation of smart phones with cameras and social media it’s not a surprise we’re hearing a lot more about these tornadoes than we have in the past. Tornadoes are nothing new here in Connecticut and even though it seems we’ve been getting an unusual amount of late this is really the way it has always been. The state is vulnerable to tornadoes and while most of our tornadoes are weak (EF0 or EF1) we have a long history of significant and violent tornadoes. Being able to hear about damage within moments and get pictures of the tornado shortly after touchdown thanks to everyone with an iPhone or Droid allows us to cover tornadoes and severe weather much more effectively.

While it’s been a busy year for tornadoes in Connecticut – to be honest – we’ve just been sort of unlucky. Marginal setups have been able to produce spinners. Hopefully yesterday’s was the last tornado of the year!

Updated: 2 Tornadoes Touch Down in Connecticut

Courtesy: Nate Borque / NBCConnecticut.com

Courtesy: Nate Borque / NBCConnecticut.com

A tornado touched down in Greenwich and Stamford Monday morning and the same storm produced another, stronger tornado in the afternoon in Windsor, Windsor Locks, and East Windsor.

This mini supercell was long lived. It produced a 4.8 mile EF-0 tornado in northeast New Jersey northwest of Newark Airport around 9:20 a.m., a 3.7 mile long EF-0 tornado in Greenwich and Stamford, and an EF-1 tornado in Windsor, Windsor Locks and East Windsor around 1:30 p.m.

This is how the Windsor tornado looked on storm relative velocity from the KBOX radar. You can see a briefly impressive couplet with 60 knots of gate-to-gate shear for one volume scan over Windsor/Windsor Locks right over the Bradley Connector. In this case the red shading are winds blowing to the west while the green shading is wind blowing to the east. Where the two meet is, in this case, a tornado vortex signature.

The tornado warning from the National Weather Service was issued at 1:31 p.m. following the 1:27 p.m. volume scan. The touchdown likely touched down around 1:27 p.m. on the Windsor/Windsor Locks line just east of Poquonock. The tornado moved across I-91. It’s all but certain there was a touchdown based on damage reports, video of the tornado, and the radar signatures here.

Here’s a picture from Windsor where a roof was ripped off a business on Hayden Station Road. This picture corresponds to when the velocity couplet on radar was strongest at 1:27 p.m. This appears to be near where the tornado first touched down.

windsorhaydenstation

The tornado touchdown was very close to where the radar showed the strongest rotation (at 6500′ AGL). Tornado damage occurred just 0.40 nm south of the couplet. No surprise that there was a slight offset as the updraft can be tilted and/or the resolution of velocity data is a bit coarse at this distance.

The weather pattern was supportive of tornadoes in Connecticut today. High-end tornadoes from supercells are rare but do occur with large, powerful, and tall thunderstorms here in Connecticut. Those are the storms like June 1, 2011 in Springfield or July 10, 1989 in Hamden that drop monster hail and have deep and powerful updrafts.  Just as common in southern New England and Long Island are mini supercells that develop in a highly sheared and very tropical airmass. These mini supercells can be quite shallow (i.e. not tall) and can drop brief tornadoes. Here’s the 1 p.m. proximity sounding from the RAP model at Bradley Airport.

17zrap

This is a classic mini-supercell sounding and hodograph for one of these events. Winds at the surface were backed ahead of the storm (notice the southerly and southeasterly winds near the ground), lifted condensation levels were incredibly low (700-800ft AGL), and there was approximately 500 j/kg of surface based CAPE with no CIN or low level inversion. The hodograph shows sufficient shear in the 0-1km and 0-3km for tornadogenesis.

This occurred in a highly tropical atmosphere (PWAT values in excess of 2.00″) and on the northern tip of a developing and northward moving low level jet. The nose of that low level jet produced a long enough and clockwise turning hodograph (indicative of large vertical wind shear has shown above) in the lowest 2 km of the atmosphere to support tornadoes.

namNE_850_spd_000

The top of this minisupercell was only about 35,000 feet high (unimpressive by thunderstorm standards), the storm produced little lightning, and the mesocyclone was only evident below 10,000 feet. Here’s a reflectivity loop (click to animate) showing the tornadic storm near BDL and an earlier storm near Agawam that exhibited some weak rotation produced 2 significant microbursts with hundreds of snapped/uprooted trees.

agawam_and_bdl

This area is no stranger to tornadoes. One of the most violent tornadoes to hit the state since the 1878 Wallingford tornado did so in Poquonock, Windsor Locks, and Suffield on October 3, 1979.

To my knowledge, this tornado was one of the most well documented on video and photographs in Connecticut history. Hamden, Bridgeport, Wethersfield and Greenwich were rain wrapped, the Springfield tornado passed north of the border, the weak Watertown tornado a few years back produced some funnel pictures but was generally obstructed by trees and hills. This is the first tornado I can remember in the state

Here’s a picture of the tornado going over the Connecticut River. How many of these are we going to see (remember West Springfield-Springfield in 2011???).

Rt 140 Bridge / Save Windsor Locks Facebook Page

Rt 140 Bridge / Save Windsor Locks Facebook Page

Here’s another picture from I-91 looking north. 91 from Windsor to Springfield is our new little tornado alley in New England I guess.

Save Windsor Locks Facebook Page

Save Windsor Locks Facebook Page

Originally some of the YouTube videos I saw of the tornado looked like there could have been a smaller rope tornado wrapping around the larger vortex but not so! My friend John Bagioni pointed me to a better video that’s close up and you can see what looks like a small “rope” tornado is actually nets from tobacco fields getting spun around the storm.

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For what it’s worth – here are the observations from Windsor Locks and Hartford (Brainard) around the time of the tornado touchdown.

KBDL 011751Z 33003KT 3SM RA BR BKN005 OVC013 23/22 A2998 RMK AO2 SLP150 P0101 60123 T02280217 10261 20222 57002
KBDL 011747Z COR 33004KT 3SM +RA BR BKN005 OVC013 23/22 A2998 RMK AO2 VIS 1 1/4V5 P0100
KBDL 011721Z 01006KT 1/2SM R06/2000V4000FT +RA FG BKN009 BKN025 OVC040 23/22 A2997 RMK AO2 P0039
KBDL 011651Z 30004KT 1 3/4SM RA BR BKN007 BKN025 OVC060 23/21 A2998 RMK AO2 TSE39 SLP152 P0022 T02280211

KHFD 011753Z 19009G15KT 10SM -RA BKN018 OVC029 26/23 A2997 RMK AO2 RAE01B51 SLP149 P0000 60011 T02560228 10261 20233 56004
KHFD 011708Z 16008KT 10SM BKN020 BKN033 OVC080 26/23 A2997 RMK AO2 RAE01 P0000
KHFD 011700Z 15005KT 10SM -RA SCT020 BKN033 OVC080 26/24 A2997 RMK AO2 P0000
KHFD 011653Z 14004KT 2 1/2SM -RA BR BKN012 BKN018 OVC029 25/24 A2997 RMK AO2 SLP149 P0005 T02500239

You can see that around 1:10 p.m. the winds at Bradley (northwest of where the tornado would touchdown) were out of the north while the winds south of the storm were out of the SSE. That slightly backed flow (SSE or 160 degrees) resulted in an area of rich helicity/low level shear that is confirmed by the 17z proximity RAP sounding at BDL I posted about. The “ground truth” obs match the 17z model analysis.

The earlier tornado in Greenwich and Stamford was weaker than the Windsor storm but resulted in widespread tree damage north of the Merritt Parkway.

Courtesy: NWS Upton, NY

Courtesy: NWS Upton, NY

This is the first tornado in Greenwich since the 2006 F2 tornado that tracked from Westchester County into Greenwich. This tornado was spawned from the same mini supercell that produced a tornado in northeastern New Jersey earlier in the morning. The rotation was impressive on doppler radar. Here’s a look at the TJFK TDWR.

BOGOgPfCAAAspce

 

In the grand scheme of things we’re fortunate that this tornado didn’t become a long-track tornado. There are some similarities between this tornado and the 2008 EF-2 New Hampshire tornado that was on the ground for 52 miles with a continuous damage path.

Timing Is Everything

5 days ago we turned in our rental car at the Thrifty location at the Denver International Airport. DIA is located several miles east of Denver (in the middle of nowhere, to be honest) and is really the start of the rural and flat high plains of eastern Colorado.

Yesterday, while we were covering severe weather at the station, Twitter exploded with incredible pictures of a tornado on the grounds of DIA. One of the most dramatic shots of the tornado was taken from, you guessed it, the Thrifty rental car parking lot near DIA.

DIA

 

The storm was quite photogenic and almost looked more like a landspout than a classic tornado. The funnel, thankfully, avoided the terminal though it did manage to get pretty close to the automated weather observation equipment that recorded a wind gust to 97 m.p.h. before biting the dust.

The contract weather observer in the tower at DIA gets mad props from me for the properly coded (and timely transmitted) METAR.

KDEN 182021Z 06022G28KT 5SM +FC TSRA FEW050 BKN080CB BKN180 21/06 A3003 RMK FUNNEL CLOUD B18 FUNNEL CLOUD E20 TORNADO B21 AO2 PK WND 07028/2020 LTG DSNT E-S RAB03 OCNL LTGICCG VC E TS OHD MOV E VIRGA SW P0005

+FC is not what you want to see at an airport if you’re flying. The storm also gave us a unique opportunity to look at it up close on conventional weather radar, particularly the Terminal Doppler Weather Radar at the airport (TDEN), which samples at a higher resolution than the WSR-88d.

BNEcgKXCEAIXgQy

 

The tornado was close enough and wide enough that the actual tornadic circulation produced a “donut hole” on the TDEN TDWR image. Rain or other “stuff” kicked up by the tornado is visible around the funnel itself. The WSR-88D radar from KFTG is also quite impressive though the donut hole is not quite as visible which lower resolution.

BNEbfp5CcAAThGB

 

Pretty wild stuff. If only we had flown out a few days later!